"Princess Zelda gets herself into a pickle and has to be un-pickled by the hero, who is called 'Link' on the few occasions when I feel mature enough to not abuse the 'Enter Your Name' Feature, and 'Fagballs' at all other times."
For starters, any video game that has a model viewer with a rotatable axis is going to be prone to this. It's meant for players to admire the graphical details, but hooboy, did some players take that to the next level. Somegames try to avert this by restricting or outright disabling rotation of the models on certain axes.
WarioWare DIY lets you make your own "microgames", and it wasn't long before people made ones themed after getting boners.
If you have a Pixiv account, you can see the sheer variety of porn microgames out there for the Japanese version Made in Ore.
One player made a subversion of these sorts of games. The microgame features a cute Moe girl with two hotspots—her breasts, and her hand. Grope her... and she gets crazy mad at you, resulting in a failure. Touch her hand, and she gives you a cheerful high-five—success!
This rule has been acknowledged for the Spore creature creator. It made the game's online feature family-unfriendly because players got to see alien races by other users, but didn't get to choose which ones they see. To put it another way, say hello to walking penises, crawling penises, flying penises, monsters with penises, penises with penises, and creatures in the form of two people copulating. And the occasional breasts.
You can forbid monsters that are risqué or you just don't like from appearing on your computer. If enough people ban a monster, it will only be used by the creator (local host only). You can also turn off sharing for monsters that are walking cocks if you create one.
This is so prevalent it has its own name - "sporn", a portmanteau of "Spore" and "Porn".
However, this sort of thing went down significantly after the patch which removed the achievement for getting one of your creations banned from the server for content violations. How they originally failed to see the obvious potential consequences of such an achievement is a mystery for the ages.
With the 'boolProp testingCheatsEnabled true' command it is possible to make any adult Sim pregnant by any other Sim. That includes getting pregnant by relatives, male pregnancy, getting pregnant by children...
Fat admirers have created patches that make the women fat.
Just to show that the immature mind will make use of the smallest degree of creative freedom, a popular clan-tag is [~c=8] (Spoilered to protect innocent eyes from the horror of a crudely drawn ascii phallus.)
Crioca: When the bag was full I went on a rampage, smacking people down with a bag full of dicks. It took me a while but eventually I thought "What the FUCK am I doing?"
Broseph Stalin: Broseph Stalin's Dorf immersion scale: T/2 where T is Time in minutes it takes you to remember hitting people with severed penises is wrong. My rating? 17.
Even without modding, it is not uncommon for players to coordinate their dwarves to create anatomically-correct monuments.
One player in an unmodded game captured a goblin-raised dwarf in a cage trap during a raid, looted their belongings, and mounted the cage in the dining hall for decoration... only to realize later that the dwarf was a female, and he'd created a "dwarven strip club". Upon relating this to Bay 12 forums, the discussion almost immediately went to the location of beards on female dwarves.
Neverwinter Nights saw some of this, ultimately driven off the giant parody-shaped cliff with a module titled "Sex and the Single Adventuress" (there is a Neverwinter Nights 2 remake as well). No matter what she wears, the provided character always has exposed breasts with simple jiggle physics modeled in, and the whole plot involves sleeping with every male you find in the game. Given that character models cannot actually touch meaningfully outside of combat, the entire area while the scene happens is blacked out with outlandish noises thrown in.
There actually is a hakpak that adds sexual poses and animations to the game called the "Intimate Animations Suite" (Page lacks screenshots, so it's sort of SFW). They can be scripted into cutscenes by module creators. The module "Save The Princess!" uses this hakpak.
Like all games, Neverwinter Nights can be modded, but out-of-the-box it comes with a feature for creating your own campaign or even making an altered version of the default campaign.
There was a Gameshark code for WWF War Zone which enabled female wrestlers to wrestle topless. They even had nipples.
Griefers have taken advantage of this as well by using images from shock sites. They usually just Squick their own team.
This trope is the reason that, ever since the SmackDown vs. Raw games have had online play capability, they have prohibited logos from being placed on the lower bodies of created wrestlers.
Dead or Alive has always featured some pretty revealing outfits, but Dimensions actually included a "Showcase" mode, which allowed you to pose "figurines" of all the fighters in various locations and positions and take pictures of them. In full 3D. This mode actually got the game pulled before release in Sweden due to backlash over the possibility of taking racy pictures of some of the underage fighters.
The Japanese version Bloody Roar 3 infamously had a code where a player could enter debug mode, which allowed the player to pose the game's models however they wanted. Enter screenshots of the female characters in some rather risque positions. The code to enter the mode was removed in the American and European releases of the game.
The Oregon Trail gave players the ability to put custom messages on tombstones for other settlers to find whenever one of them died (usually from dysentery). Apart from a few with "Pepperoni and cheese", a Shout-Out to an old Tombstone Pizza commercial, most ended up with obscenities.
Yes, there are plenty of Knights of the Old Republic mods where the fugly underwear is swapped out for something a little easier on the eyes. Nude Bastila and Handmaiden mods are easier to find, but there are just as many for the guys. The nude male mods, though, tend to be passed around on invite-only forums and lists.
Skyrim in particular has among the most realistic and titillating looking nude mods (both female and male, and both armored and unarmored), especially for those who are able to set their game's graphics details to the maximum (so it can be viewed up close with the free viewing camera, great for those who want to take screenshots), and one mod site, Skyrim Nexus, has plenty to choose from (fortunately such mods hosted there have restricted access, requiring registration and opting to be able to view adult content). Only a portion of those mods are adult, but there ARE a lot of them.
And to top it all off, a group of Skyrim modders came together and created the Sexlab mod, which is in fact a large and robust library of animation scripts for all sorts of acts of sex, along with some voice files, and it even includes a configuration menu. This would allow modders making sex related mods to focus on their mod's themes (which could range from combat sex and assault, to slavery and slave trade, to mere consensual sex between lovers) without having to create their own animated sex scenes, instead referring to the library for all that. For all its intents and purposes, one can't help but hand it to these guys for this level of Genius Programming.
Call of Duty: Black Ops: It is possible to create your own emblem to put onto your gun. Of course, since you can make any shape with some imagination, it doesn't take a genius to see where this went...
Thus leading to the horrible humiliation of getting killed over and over by a guy whose clan tag is [c==8] and who has a dick as his emblem.
And some people have made naked character skins and have modded the ingame paintings into porn.
In many of the Total War games, it's quite easy to turn your royal family into an incestuous nightmare with siblings marrying each other and elderly uncles marrying their relatives the moment they come of age. The first Medieval game will automatically create father/daughter pairings for you with surprising frequency. Particularly Squicky (even comparatively) is when the age of the father creates the horrible realization about the maximum possible age their child can be. You can make your own father-daughter pairing as well - wait until you get a new king, and then marry him to a princess. She'll register as his daughter.
The M.U.G.E.N engine makes it possible to import character bootlegs from any 2D fighting game. There are enough nude retextures, some with x-rated animations, to repackage into a 2GB partial conversion. And then there are maps with names like 'Palace of Porn', filled with scans of hentai and porn.
The Slime Original Character. All of its attacks transform it into a long appendage that goes to some very sensitive places. And it turns into a nude woman when struck.
As with "The Old Gods" DLC, it is possible,and even encouraged, to marry close kin if your religion is Zoroastrian. This DLC also added concubines, which are sex slaves for all purposes and intents.
Garry's Mod, a famous Half-Life 2 sandbox mod, allows you to build contraptions using models from assorted Valve games to do obvious things such as making giant dongs or poorly putting ragdolls into sex poses. The game's official site used to be littered with pornographic contraptions and images before constant complaining from the Facepunch community led to them being cleaned up. The pornographic uploads are borderline extinct on Facepunch. Other sites? Not so much.
Ever played the multiplayer of good ol' Half-Life? Servers got overrun by players with... interesting names, including gems like "Condomising Proorgasmer", "Lesbian Syphilitic Spanker", and "Prostituted Paderastive Freak".
It's easier to count the non-pornographic mods for Dragon Age: Origins than it is to count the porn ones. Ranging from the typical nude skins, to fully modeled penises for the men that you can select length, circumcision, erect or non erect, penises for the monsters, the dog, the female monsters, replacing the current tame sex scenes with graphic hardcore sex, adding new graphic hardcore sex and more.
Heartache 101 Sour Into Sweet lets the player mod a Romance Game with graphics, storylines, and music. It may be easier to create NSFW content with Heartache 101 rather than creating a unique game with Renpy or other tools.
It is very common to see a phallus-shaped city in Carcassonne, though this is (usually) due to simple happenstance with what tiles get drawn.
Yahtzee: I did check and it is extremely possible to make a Mii that has what looks like a cock for a face. It's equally possible to name him "Seńor Coquonface" and make him your StreetPass ambassador who greets everyone in the vicinity with the phrase "I'm watching you!" in block capital.
Mario Kart DS had a feature that lets players create an emblem to display on their karts throughout the game, including multi-player and online. Didn't take long for nearly every player to decorate the hood of their karts with breasts and penises. You could also change your profile name, which lead to some "creative" names online.
Mario Kart Wii and Mario Kart 7 introduced global online leader boards where you could show your custom Mii and his custom name to the world. Kinda makes you chuckle/lose hope for humanity when the guy with the fastest time in a weekly contest is called "Penis Man!" complete with penis face.
3D Custom Girl is a "game" (more like a Virtual Paper Doll, except it's in full 3D) where you don't even have to mod anything to have the characters strip down to their panties (or naked) or make sexy poses. The game lets you do all this right from the get-go.
Hyperdimension Neptunia allows players to use pictures stored in their PlayStation 3 units for "R/W disc" special attacks — even pornographic ones. Hyperdimension Neptunia mk2 allows Nepgear's CPU outfit to be customized via the Costume Canvas system, and "nude mods" have been made. Hyperdimension Neptunia Victory extends the Costume Canvas system to the other CPUs and the CPU Candidates, allowing even more perverted potential.
Out of all the nude mods, few got the game's developer's attention other than that of Tomb Raider, resulting in the shutdown of the "Nude Raider" website.
In the early days of Arcade Games, high score screens would ask you to enter your initials after getting a high score. Naturally, some people would enter ASS, SEX, FUK, or the like. Most later arcade machines have code to block such initials.
This is discussed (but not directly shown) in the Friends episode "The One Where Joey Dates Rachel," where Chandler plays Pac-Man and fills the high score table with dirty words:
Monica: You think this is clever? Chandler: Well, you know, they only give you three letters, so after A-S-S, it is a bit of a challenge.
Not that they could show it — Pac-Man only saved one high score and never asked the player for initials.
Widely rumored to be the reason Pac-Man's name was changed from Puck-Man in Japanese to Pac-Man in English. Midway realized the potential of disgruntled players scraping off part of the 'P' to change it into an 'F'.
The Soul Calibur series, specially the fourth installment, had some pretty risque outfits for characters, whether they be original or a custom look for already existing characters, which meant you could have some rather stripperiffic looks for any of the characters.
Soul Calibur 5 has "Specialised equipment" for the character creator, which can be placed anywhere on the body, and includes things like horns and arrows. Guess where some trolls like to place these things.
Darkstalkers 3 features a character palette editor. Since Felicia is already technically fully naked, it doesn't take much to change her white fur into the same color as the rest of her skin, and the name editor allowed quite a bit of space to name your new naked catgirl something fairly lewd as well.
Ultimate Marvel Vs Capcom 3 featured a slightly less extreme example. The character model viewer allows players to control the X, Y, Z, and rotational axis in order to examine all the characters during their default fighting stance. Felicia is the example once again, as a player can zoom in quite far to take a really close look at her underside... or her massive breasts as they jiggle in all her idle-stance glory.
Animal Crossing games have a character named Blanca who will occasionally show up in the player's town. Players are free to give her a face if she doesn't have one, or "rearrange" her face if she does. Now, faces that Blanca already has are downloaded from Nintendo's servers and faces that any player draws are inadvertently uploaded to Nintendo's servers, so long as the console has internet connectivity. The game also doesn't check to see if the player drew on a valid face, and Nintendo doesn't do random sampling checks on the faces uploaded to the servers. Net result: Blanca can shown up to your town a few dozen times. With a penis doodled onto her face.
Animal Crossing also allows for custom designs, and flags are one. Welcome to perverted designed flags and paintings.
Isabelle ended up becoming fuel with her level of Moe.
One recent inversion is something called Joji Fuku O Kite (Little Girls Wearing Clothes) which was made from the old unlicensed nudie game Idol Shisen Mahjong featuring pictures of women who strip for the player between levels. Idol Shisen Mahjong in turn is actually an early hack of Tiles of Fate, in which the prize for beating levels was being shown conquering various parts of China. In Joji Fuku O Kite, the prize is... well, pictures of fully clothed little girls. They don't even pose pruriently or anything.
In Saints Row The Third, you can make your character have ridiculously large breasts or dick, depending on his/her gender. If you create a Fem!Boss, you can get piercings that cover her nipples, and a pair of underwear (that you change to match the color of her skin), so you can now have her run around town 99.9% completely naked. Alternatively, you can get a standard nude mod to remove the mosaic and add textures. Enjoy, pervert.
Rest assured, when Super Scribblenauts says you can create anything, it means it, particularly when the word "nude" is an accepted adjective (although it only colors the object the same color as caucasian skin).
Scribblenauts Unlimited is worse, since you can take various pieces of different objects and assemble them into ANYTHING. Yes, you can attach that hot dog where you're thinking of (and recolor it to match the character's skin).
Even internal development teams are not immune. When Intellivision programmers working on TRON: Solar Sailer noticed that the Intellivoice module had an odd way of pronouncing the vowel in "can't," they decided to use the four-letter word in an unreleasable joke game, which consisted of a talking title screen pasted on to a hacked version of Astrosmash with the missile launcher and flying saucers replaced by genitalia.
In-fiction example, though not quite a video game: Snow Crash mentions that one can make one's Metaverse avatar a giant talking penis, and that people have done so.
In Lollipop Chainsaw, you can actually get an achievement for trying to look up the player character's miniskirt.
In the development stages of Halo 3, the possibility of letting players draw their own emblems was considered. However, the idea was quickly scrapped when the dev team found themselves branding their characters with badly drawn penises. Now emblems can only be made of predefined shapes.
You can add a D/s twist to any dating sim or visual novel with a faceless protagonist simply by naming them "Master".
In the NES game "The Last Ninja" it is possible for the player's (male) ninja to enter the women's washroom. If you move around and click randomly (you can't see where they are moving past the door, but the camera moves around indicating they are moving) you may eventually even get a powerup, after hearing a toilet flush. Take from that what you will.
In the Homebrew scene for the Xbox 360Kinect, if you know where to look, there exists games where you can reach out and manhandle cute girls using the Kinect's ability to track your hands and body movements.
Second Life is full to the brim with the trope. Being an MMO styled game with heavy customization, you'd be lying to yourself if you claim to have never seen another player with a huge dong flopping about on his avatar.
Pokémon games manage to have some of this, despite Nintendo's censorship and language filters. For example:
In Ruby, Sapphire, and Emerald, some wags managed to make racy phrases with the prefabricated wordlist.
A age-old Double Entendre goofery with Pokémon of phallic/yonic design with moves like Harden, Lick, Bounce, Pound, and other such moves that could be questionable in context of their combinations, topped off with creatively suggestive nicknames.
In X and Y, some particularly creative players have abused the PR Video feature in Lumiose City to have mock Immodest Orgasms. They have also exploited the free-rotating character models of the Pokémon in the Pokédex and petted their Pokémon in inappropriate places in Pokémon Amie.
The BBC Micro game Frak! was hacked into an infamous "rude version" which substituted two letters in the title, changed the main character's name to "The Rapist" and his Killer Yo-Yo to... well, guess.